A film known from U.S. Pat. No. 3,741,253 to Brax comprises a barrier layer of a vinylidene chloride copolymer (saran) between a layer of ethylene-vinyl acetate copolymer and layer of a cross-linked ethylene-vinyl acetate copolymer. Vinylidene chloride copolymers are known barrier materials to fluids such as oxygen. Optionally, the film is oriented. As disclosed in U.S. Pat. No. 4,064,296 to Bornstein the barrier layer may also be a hydrolyzed ethylene-vinyl acetate copolymer (EVOH). It has similar oxygen barrier properties as vinylidene chloride copolymers and offers the advantage that it may be irradiated without discoloration, which is further discussed below. Optionally, the film is oriented.
Blends of linear low density polyethylene and ethylene vinyl acetate copolymer in oriented barrier films are disclosed in U.S. Pat. No. 4,457,960 to Newsome, which claims an oriented multiple layer polymeric film, comprising (a) a first barrier layer, said first layer having two opposing surfaces; (b) a second layer adhered to one said surface, said second layer being 10% to 90% linear low density polyethylene and 90% to 10% ethylene vinyl acetate; and (c) a third layer adhered to the other said surface, the composition of said third layer being selected from the group consisting of (i) ethylene vinyl acetate, and (ii) blends of 10% to 90% linear low density polyethylene with 90% to 10% ethylene vinyl acetate.
U.S. Pat. No. 4,755,402 to Oberle discloses a multi-layer heat-shrinkable (oriented) film comprising a barrier layer of polyvinylidene chloride copolymer and an outer surface abuse layer of polyamide directly adhered to the barrier layer.
U.S. Pat. No. 4,640,856 to Ferguson et al, commonly assigned to W. R. Grace, discloses a three-layer, thermoplastic barrier film of the structure: (a) a layer of very low density polyethylene having a density of less than 0.910 gms/cc; (b) a barrier layer selected from the group consisting of: (1) copolymers of vinylidene chloride and (2) hydrolyzed ethylene-vinyl acetate copolymers; and (c) a thermoplastic polymeric layer, said layer being on the side of the barrier layer opposite to that of layer (a).
It is an object of the present invention to provide a packaging film, preferably a barrier film, and bags made therefrom which have excellent or improved heat shrink characteristics over those of the materials used in the past, such as those of U.S. Pat. No. 4,640,856. This means that the shrink energy (often called shrink tension) and modulus are higher. These properties may be measured in accordance with ASTM D-2838 and ASTM D-882 Method A, respectively. It has been unexpectedly found that by including an interior layer of polyamide or polyester disposed between the sealing layer and another layer of a shrink film, the resultant film will have a higher shrink energy and higher modulus. Polyamide and polyester are stiffer than the ethylene polymers typically used in shrink films. Since stiffer materials are employed, difficulty in stretching during orientation to make the shrink film would be expected. It is an advantage that the stiffness helps keep unsightly purge from accumulating when the films are used in the shrink packaging of meat.